This disclosure generally relates to a CVD (chemical vapor deposition) apparatus used for forming a film such as a silicon oxide film or a silicon nitride film on a substrate held on a boat at elevated temperatures in a processing space filled with a gas mixture of two gases of different kinds, such as a silane family gas and an N2O gas, under a reduced pressure environment.
First, a low-pressure CVD apparatus (referred to hereinafter as LPCVD) according to a related art of the present invention will be explained.
FIGS. 8A and 8B shows an example of a CVD apparatus according to a related art, wherein FIG. 8A shows the overall construction of the CVD apparatus, while FIG. 8B shows the CVD apparatus in an enlarged cross-sectional view taken along a line C-C′ of FIG. 8B.
Referring to FIGS. 8A and 8B, the CVD apparatus includes, at a central part thereof, a boat 104 for holding substrates 102, which may be a semiconductor substrate, on which film formation is to be made. The boat 104 is supported on a flange 110 having a circular shape in a plan view, and thus, the bottom part of the boat 104 is supported on the flange 110.
Further, an inner tube 106 of cylindrical shape is disposed so as to surround the boat 104, wherein the inner tube 106 is held by an inner tube holding part 110a of the flange 110 provided for holding the inner tube 106. Here, the inner tube holder part 110a is a plate-like member projecting from the inner surface of the flange sidewall inward with a predetermined separation from the bottom surface of the flange 110.
Further, there is disposed an outer tube 108 at the outside of the inner tube 106 so as to enclose the inner tube 106 entirely, wherein the outer tube 108 is held by an outer tube holding part 110b of the flange 110.
Further, there are provided two gas introducing nozzles 132a and 132b on the flange sidewall at a height between the bottom surface of the flange 110 and the inner tube holding part 110a of the flange 110, wherein the gas introducing nozzles 132a and 132b are provided respectively for introducing a silane gas and an N2O gas into the inner tube 106. Thereby, the as introducing nozzles 132a and 132b are provided such that respective gas ejection ports protrude inward from the inner surface of the flange sidewall, in other words, the inner surface of the sidewall of the flange 110, with a distance of about several centimeters.
Further, there is provided a gas evacuation port 116 on the flange sidewall in the part between the inner tube holder part 110a and the outer tube holder part 110b, wherein the gas evacuation port 16 is connected to a pump (not illustrated), and the gas inside the outer tube 108 is evacuated therefrom through the gas evacuation port 116 by the pump.
With the CVD apparatus of FIGS. 8A and 8B, two gases ejected respectively from the gas introducing nozzles 132a and 132b are mixed with each other inside the inner tube 106 and film formation is conducted on the surface of the substrates 102.
More specifically, the gases introduced from the gas introducing parts 132a and 132b reach a substrate 102 held in the boat 104 at the bottom part thereof and moves through the inner tube 106 in the upward direction. The gas reached the top part of the inner tube 106 is then evacuated from the gas evacuation port 116 to the outside of the apparatus by the pump after flowing through the space between the inner tube 106 and the outer tube 108.
With this CVD apparatus, it should be noted that the gas introducing nozzles 132a and 132b are disposed so as to project for a distance of several centimeters from the inner surface of the flange sidewall toward the central part of the flange 110 for facilitating mixing of the two gases thus introduced therefrom.
Further, as shown in FIG. 9, there is proposed a construction in the related art in which an L-shaped gas introducing nozzle 134 is provided such that the gas ejection port of the nozzle 134 points upward to the ceiling of the outer tube 108. With this construction, the gases are injected in the upward direction for facilitating mixing of the two gases. Reference should be made to Patent Reference 1, for example.
With such CVD apparatuses of FIGS. 8A and 8B, however, there remains the problem that mixing of the gases tends to become poor and non-uniform in the vertical direction, and thus, there has been the need of providing a temperature gradient in the vertical direction along the boat 104.
In order to attend to the foregoing problem and to facilitate mixing of the two gases, there is a proposal to provide four nozzles for the gas introducing nozzles, such that two of the gas introducing nozzles are provided so as to project from the inner surface of the flange sidewall with a distance of several centimeters and such that the remaining two gas introducing nozzles are formed to have the L-shaped form pointing the ceiling of the outer tube. Thereby, the parts pointing the ceiling of the outer tube extend to the top part of the boat and the gas ejection port is formed at a level corresponding to the top part of the boat. Reference should be made to Patent Reference 1, for example.
With the CVD apparatus shown in Patent Reference 1, on the other hand, the two nozzles projecting from the sidewall surfaces of the flange toward the flange central part are used solely for the film formation on the substrates placed in the lower part of the boat, while the L-shaped nozzles are used solely for the film formation on the substrates placed in the upper part of the boat.
PATENT REFERENCE 1 Japanese Laid-Open Patent Application 1-251725
In such a LPCVD apparatus of such vertical type, it is generally easy to carry out film formation on the substrate surface with an in-plane uniformity of 3% or less in terms of film thickness. On the other hand, in the case of forming an oxide film on a substrate surface by mixing a silane family gas and an N2O gas, it has been difficult to attain the in-plane uniformity of smaller than 5% because of poor gas mixing at the bottom part of the boat.
Because Patent Reference 1 has the main objective of attaining uniformity of film thickness between the top part of the boat and the bottom part of the boat, the LPCVD apparatus uses the same nozzles that are used in the conventional LPCVD apparatus of vertical type at the bottom part of the boat. Thus, even when there is provided improvement to use the L-shaped nozzles, the in-plane uniformity of film thickness is not improved for the substrates placed at the bottom part of the boat.
Because there are cases in which the films thus formed by the CVD process are used for capacitor elements, there arises a problem that the variation of film thickness and film quality affects directly on the performance of the capacitor element in terms of the variation of the capacitance value.